The control of hard roof, the prevention and treatment of coal and rock burst, the gas extraction in coal seams with low permeability, the prevention and treatment of coal and gas outburst, and fully-mechanized top coal caving in hard and thick or extremely thick coal seams are technical problems faced in underground coal mining. Transforming the structure of coal and rock masses is the core issue in solving all of these problems, by increasing the number of hydraulic fractures in the coal and rock strata, their strength can be weakened and permeability can be improved. At present, hydraulic fracturing is an effective method to realize the structure transformation of coal and rock masses.
The existence of the complex structure of coal and rock strata, soft coal seam, adsorption and desorption effect of methane, developed natural fractures, anisotropy of coal seam and the influence of mining work result in the hydraulic fracturing of coal-rock mass becoming complicated, which has been proved by underground coal mine hydraulic fracturing and coal-bed-methane exploitation.
The propagation of hydraulic fractures and their morphology are complex in hydraulic fracturing. Real-time propagation and morphology of hydraulic fractures cannot be monitored precisely at present. The propagation law of hydraulic fractures in site is not revealed, which result in the blindness and poor effect of hydraulic fracturing. The development and popularization of hydraulic fracturing technology are restricted. Meanwhile, there is no a complete set of experimental system in coal industry to study these problems systematically and thoroughly. It is urgent to develop a complete set of experimental system to study the mechanism of the above problems.